Reduction of an ester functional group to the corresponding alcohol is one of the fundamental reactions in organic chemistry, and is used in a large number of chemical processes. In general, two main types of processes are known to achieve such a transformation. Such types of processes are the following:
a) hydride processes, in which a silyl or metal hydride salt, such as LiAlH4, is used;
b) hydrogenation processes, in which molecular hydrogen is used.
From a practical point of view, hydrogenation processes are more attractive as they can be run using small amounts of catalyst (typically 10 to 1000 ppm relative to the substrate) and in the presence of small quantities or even in the absence of solvent. Furthermore, hydrogenation processes do not require the use of highly reactive and expensive hydrides, and do not produce important amounts of aqueous waste.
One of the mandatory and characterizing elements of hydrogenation processes is the catalyst or the catalytic system which is used to activate the molecular hydrogen in view of the reduction. The development of useful catalysts or catalytic systems for the hydrogenation of an ester functional group represents still an important need in chemistry.
Amongst the few catalysts or catalytic systems known to perform such reductions one may cite the ruthenium/phosphine complexes, obtained by the reaction of ruthenium oxide or carboxylate precursor with a mono-, di- or tri-phosphine ligand (an example of which is described by Elsevier et al. in Chem. Commun., 1998, 1367). In this type of complex the ruthenium metal is coordinated only by “acac” ligands and phosphine atoms, limiting thus the diversity of the ligand structure and coordination sphere around the metal center. As a consequence of such little diversity the tuning of the activity and of the performance of the hydrogenation process is not easy. Furthermore, the experimental conditions require very high pressures (at least 70-130 bars) and temperatures (120-180° C.).
Therefore, there is a need for hydrogenation processes using alternative catalysts or pre-catalysts, preferably having a greater diversity in the ligand structures and coordination spheres around the metal center and allowing the use of softer experimental conditions.